Modulated thermal conductance thermal enclosure

ABSTRACT

A thermal insulation device includes a first plate, a second plate formed to nest adjacent the first plate with a gap between the first and second plates, a porous material disposed in the gap between the plates, a sealing layer disposed between the first and second plates such that the porous material is sealed from ambient at a pressure less than ambient, and a vapor generating material disposed in the gap.

RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 62/356,627 (entitled Modulated Thermal Conductance ThermalEnclosure, filed Jun. 30, 2016) which is incorporated herein byreference.

BACKGROUND

Conventional thermal insulating materials (e.g. polyisocyanurate,polystyrene, polyurethane) cannot meet the thermal resistancerequirements of certain applications requiring thin, high performancethermal insulation (e.g. portable fuel cells). Vacuum-based thermalinsulators (e.g. vacuum insulated panels) can meet the thermalperformance requirements of these applications, but cannot becost-effectively manufactured in custom form factors.

Some devices, such as power sources and sensors must operate over abroad range of ambient temperatures. Thermal insulators may be incapableof maintaining such devices within their operating range in the face ofsuch a broad range of ambient temperatures.

SUMMARY

A thermal insulation device includes a first plate, a second plateformed to nest adjacent the first plate with a gap between the first andsecond plates, a porous material disposed in the gap between the plates,a sealing layer disposed between the first and second plates such thatthe porous material is sealed from ambient at a pressure less thanambient, and a vapor generating material disposed in the gap.

A thermal insulation enclosure includes a first plate, a second plateformed to nest adjacent the first plate with a gap between the first andsecond plates, a porous material disposed between the plates, a vaporgenerating material disposed in the gap, a sealing layer disposedbetween the first and second plates such that the porous material issealed from ambient at a pressure less than ambient, a duplicate set offirst and second plates having a porous material, vapor generatingmaterial and sealing layer formed to mate with the first and secondplates to form a chamber, and a device disposed within the chamber thatis thermally insulated from ambient by the enclosure.

A method includes pressing a porous material between two plates suchthat the plates are separated from each other by a gap defined by theporous material, including a vapor generating material in the gap, andin a partial vacuum, depositing a conformal sealing layer to cover theporous material in the gap between the two plates to form a gas seal ofthe porous material and vapor generating material from ambient andmaintain the partial vacuum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram cross sectional representation of a thermalinsulator according to an example embodiment.

FIG. 2 is a chart illustrating calculated gas conductivity for air as afunction of gas pressure for different characteristic system sizeaccording to an example embodiment.

FIG. 3 is a slide providing further description of vapor generatingmaterial, and example test results.

FIG. 4 is a graph illustrating pressure versus temperature for twomaterials.

FIG. 5 is a block diagram representation of how two sets of plates aresealed to form an insulating enclosure around a device according to anexample embodiment.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments which may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that structural, logical andelectrical changes may be made without departing from the scope of thepresent invention. The following description of example embodiments is,therefore, not to be taken in a limited sense, and the scope of thepresent invention is defined by the appended claims.

A vacuum-based thermal insulator provides thermal resistancerequirements of applications requiring thin insulation, and can be madein a wide range of custom form factors. In one embodiment, an improvedhigh performance thermal enclosure includes a feedback mechanism whichmodulates its thermal resistance based on ambient temperature. At verylow ambient temperatures the thermal resistance is high, while at highambient temperatures the thermal resistance is low, enabling a deviceutilizing the enclosure to maintain a temperature which is within thedevice's operating range.

FIG. 1 is a block diagram cross section representation of a thermalinsulator 100. The thermal insulator may be formed for an outside plate110 spaced from an inside plate 115 by a nano-porous material 120. Inone embodiment, the plates may be similarly shaped and of slightlydifferent sizes such that the inside plate 115 may nest inside theoutside plate 110, creating a space between them that the material 120occupies. In one embodiment, the space may be fairly uniform between theplates.

In one embodiment, the plates have sides indicated at 125 and 130 thatextend away from generally planar portions of the plates. The distancebetween the sides 125 and 130 may be the same as the distance betweenthe generally planar portions of the plates, or may vary in furtherembodiments. The shape of the generally planar portion of the nestedplates may be circular, oval, rectangular, or any other shape desired,such as a polygon. The sides of the plates extend along the entireperimeter of the generally planar portions. In still furtherembodiments, the generally planar portions of the plates may be curvedas opposed to planar. Note that while the distance between the platesand sides of the plates is substantially equal in some embodiments, thedistance may be varied in further embodiments where substantiallyuniform insulation is not needed.

A portion 140 of the sides of the plates is illustrated in furtherdetail in a blown up view also at 140. The blown up view of portion 140illustrates a sealing layer 150 that helps maintain a vacuum within thenano-porous material 120. In one embodiment, the sealing layer 150includes a layer of a polymer 155, such as parylene or other lowthermally conducting material and a metal layer 160 of low thermallyconducting metal, such as aluminum or NiCr for example. A furtherpolymer or other layer may be included in further embodiments.

In one embodiment, the polymer or plastic layer 155 may be betweenapproximately 50 to 200 um thick. The metal layer may be approximately80 nm thick. The purpose of the sealing layer 150 is to help maintain avacuum, which may be simply a low pressure as opposed to an absolutevacuum, within the space between the plates. Thus, the thickness of eachlayer may be varied based on the material used to maintain the vacuumfor a desired length of time. Since the metal layer may be morethermally conductive, it is desirable in some embodiments to utilize ametal and a thickness of the metal that minimizes its thermalconductance between the plates. The vacuum provides an area of lowthermal conductance, K. As such, it may be varied between absolute andambient pressure depending on the overall thermal properties desired.Ambient pressure may correspond to atmospheric pressure, which may varywith weather conditions and altitude or depth. In one embodiment, thevacuum is kept between 0 and 100 Pa (Pascal—Newtons/Meter²). Note thatthe portion 140 shown is provided for illustration of the sealing layer150 and may not be reflective of the actual shape of the portion 140.

In one embodiment, the material 120 may be a low-density (200-250kg/m{circumflex over ( )}3) mixture of fumed silica, fiberglass, andsilicon carbide (and optionally getter materials to getter gas resultingfrom outgassing or leakage through the seal) may be pressed into acustom form factor enclosure, such as the two nesting plates 110 and115. The fumed silica mixture fills the gap between the two nestedplates that comprise an enclosure. The mixture is a nano-porous opencell material in one embodiment such that a significant portion of thevalue occupied by the material is open, as opposed to closed cellmaterials. A small gap thermally isolates the two plates; this gap ismay be coated with a thin, low-thermal conductivity material (ormaterials) and forms a gas seal as indicated by sealing layer 150.

The space between the plates is evacuated, forming an enclosure withvery high thermal resistance between the inner and outer plates. In oneembodiment, a device 170, such as a fuel cell based power generator, canbe placed within a pocket 175 created by two enclosures 180 and 185, andprovide very high thermal resistance between the interior of the pocketand ambient environment. In the case of some fuel cells, the twoenclosures may not be sealed together, to allow at least oxygen,indicated by arrow 190, from ambient to reach the device 170 foroperation of the device. In further embodiments, where access to ambientis not needed, the enclosures may be sealed together by glue, welding,clamping, or other means of attaching the enclosures together.

FIG. 2 is a chart illustrating calculated gas conductivity for air as afunction of gas pressure for different characteristic system sizeaccording to an example embodiment. The input temperature was 20° C.,and the pore sizes are indicated by format of the line at the top of thefigure, ranging from 10 nm to 100 mm. Note that the use of pores in thenanometer range allow operation at higher pressures for a same level ofthermal conductivity, which may be easier to achieve and maintain overlong periods of time.

In some embodiments, a vapor generating material is included in thematerial 120. The vapor generating material may have a pressure vs.temperature relationship that modulates the thermal conductance of thevapor in the gap between the enclosure plates, over a desiredtemperature range.

The vapor generating material may be selected such that vapor pressureis in a desired range (e.g. 100-1000000 Pa for Nano-porous silica) whichmodulates the thermal conductivity of the vapor over the desired ambienttemperature range. Example materials include cyclohexane or water vapor.Different vapor generating materials may be selected depending on thepore size of the silica. Low density, small pore size materials otherthan silica may also be used if a different thermal resistance value ortemperature range is desired. Other materials may include Isopropanol,1-Butanol, Cyclohexane, Ethanol, and Ethyl acetate. In some embodiments,only one of the example materials or still other materials whichgenerate vapor at a desired temperature is used.

For any particular embodiment, the vapor generating material may beselected on the basis of the desired temperature range of theapplication and the pore size of the powder. By matching an overallchange in pressure for the desired temperature range with the pressurerange that creates the largest change in gas conductivity at a givenpore size (FIG. 2), the design can be optimized for a given application.

A method for forming a thermal enclosure is as follows:

1) Mixing a fumed silica, silicon carbide, fiber glass, and optionallygetter material to create a nano-porous material. Note that such mixingis well known in the art as described in at least three papers, such asDry Powder Processing of Fibrous Fumed Silica Compacts for ThermalInsulation Hiroya Abe,*,w Isami Abe, Kazuyoshi Sato,* and Makio Naito*2005; Experimental characterisation and evaluation of thethermo-physical properties of expanded perlite—Fumed silica compositefor effective vacuum insulation panel (VIP) core M. Alama, H. Singha,*,S. Brunnerb, C. Nazirisa 2015; Performance properties of vacuuminsulation panels produced with various filling materials Metin Davraz*and Hilmi C. Bayrakci 2014.

In one embodiment, the mixture is composed of 70-90% fumed silica ofapproximately 10 um grain size, 1-10% SiC powder of approximately 0.5 umgrain size, and 5-15% glass fibers, 1-2 mm×10 um. These are mixedmechanically at low speed (<1000 rpm) for several minutes.

2) Pressing the silica mixture between two plates that comprise theenclosure.

3) In a partial vacuum (<1000 Pa), deposit a conformal coating (e.g.10-100 um of a polymer such as parylene) to cover the silica in the gapbetween the plates, forming a gas seal.

4) In a partial vacuum (<1000 Pa), deposit a layer of metal (e.g.10-1000 nm of Al, NiCr) to cover the parylene.

5) Optionally repeat the polymer/metal coating process to create amulti-layer seal which further reduces permeability (increases lifetime)

The fumed silica mixture in one embodiment may be was 2/88/10% SiC/FumedSilica/Glass fiber.

6) Optionally, add the vapor generating material to the gap.Alternatively, the vapor generating material could be added during theinitial polymer coating process, by filling the deposition chamber withthe desired material (provided it doesn't interfere with the depositionprocess). The vapor may also be introduced via other means, by forexample breaking a capsule of the material within the gap, after theseal has been deposited. The capsule may be broken by any means thatdoes not adversely degrade the seal, such as by sound waves or heat.

In some embodiments, the resulting adaptive insulation based onnano-porous silica provides approximately 10 times the thermalresistance of conventional insulation.

The use of a temperature dependent vapor pressure enables modulation ofthe thermal resistance. The thermal resistance may decrease at hightemperatures within the enclosure, allowing heat to be transferred toambient. At lower temperatures, the thermal resistance may increase.

FIG. 3 shows temperature of a fuel cell device versus ambienttemperature. With a low thermal conductivity insulation design, theinternal temperature of an enclosure can get too warm at high ambienttemperatures, limiting an operating range of the fuel cell. Using theadaptive insulation with an appropriate vapor pressure temperaturecharacteristics facilitates passive temperature feedback, expanding theoperating range from −30° C. to 80° C. ambient.

FIG. 4 is a graph illustrating temperature versus pressure for twodifferent vapors such as a vapor including water and a vapor includingn-C12H5.

FIG. 5 is a block diagram representation of how two sets of plates 180,185 are sealed at 510, 520 to form an insulating enclosure 500 arounddevice 170. Seal 510 represents a sealing together of outer plates ofthe sets of plates, while seal 520 represents a sealing together ofinner plates of the sets of plates. The seals 510 and 520 may beobtained via weld or adhesive in various embodiments.

EXAMPLES

1. A thermal insulation device including a first plate, a second plateformed to nest adjacent the first plate with a gap between the first andsecond plates, a porous material disposed in the gap between the plates,a sealing layer disposed between the first and second plates such thatthe porous material is sealed from ambient at a pressure less thanambient, and a vapor generating material disposed in the gap.

2. The thermal insulation device of example 1 wherein the vaporgenerating material has a pressure vs. temperature relationship thatmodulates the thermal conductance of the vapor in the gap between theenclosure plates.

3. The thermal insulation device of any of examples 1-2 wherein thevapor generating material comprises cyclohexane, 1-Butanol, Cyclohexane,Ethanol, Ethyl acetate, or water vapor.

4. The thermal insulation device of any of examples 1-3 wherein thefirst and second plates comprise a substantially planar portion andsides, wherein the sealing layer is disposed between sides of the firstand second plates.

5. The thermal insulation device of example 4 wherein the sealing layercomprises a polymer material and a metal layer disposed between thepolymer layer and ambient.

6. The thermal insulation device of example 5 and further comprising anadditional layer of polymer and metal.

7. The thermal insulation device of any of examples 1-6 wherein theporous material is a nano-porous material comprising an open cellmaterial.

8. The thermal insulation device of any of examples 1-7 wherein theporous material is a nano-porous material comprising a low-densitymixture of fumed silica, fiberglass, and silicon carbide.

9. The thermal insulation device of example 1 wherein the porousmaterial comprises a getter material.

10. A thermal insulation enclosure including a first plate, a secondplate formed to nest adjacent the first plate with a gap between thefirst and second plates, a porous material disposed between the plates,a vapor generating material disposed in the gap, a sealing layerdisposed between the first and second plates such that the porousmaterial is sealed from ambient at a pressure less than ambient, aduplicate set of first and second plates having a porous material, vaporgenerating material and sealing layer formed to mate with the first andsecond plates to form a chamber, and a device disposed within thechamber that is thermally insulated from ambient by the enclosure.

11. The thermal insulation enclosure of example 10 wherein the first andsecond plates and duplicate first and second plates comprise asubstantially planar portion and sides, wherein the sealing layer isdisposed between sides of the first and second plates, wherein the sidesof each set of plates align to form the chamber, and wherein the vaporgenerating material has a pressure vs. temperature relationship thatmodulates the thermal conductance of the vapor in the gap between theenclosure plates.

12. The thermal insulation enclosure of any of examples 10-11 whereinthe vapor generating material comprises cyclohexane, 1-Butanol,Cyclohexane, Ethanol, Ethyl acetate, or water vapor.

13. The thermal insulation enclosure of any of examples 10-12 whereinthe device within the enclosure comprises a fuel cell based powergenerator and wherein the enclosure includes a path from ambient toallow ambient oxygen to reach the device.

14. The thermal insulation device of any of examples 10-13 wherein thesealing layer comprises a polymer material and a metal layer disposedbetween the polymer layer and ambient.

15. The thermal insulation device of any of examples 10-14 wherein theporous material comprises an open cell material including a low-densitymixture of fumed silica, fiberglass, and silicon carbide.

16. A method including pressing a porous material between two platessuch that the plates are separated from each other by a gap defined bythe porous material, including a vapor generating material in the gap;and in a partial vacuum, depositing a conformal sealing layer to coverthe porous material in the gap between the two plates to form a gas sealof the porous material and vapor generating material from ambient andmaintain the partial vacuum.

17. The method of example 16 wherein the porous material comprises amixture of fumed silica, fiberglass, and silicon carbide, and whereinthe vapor generating material comprises cyclohexane, 1-Butanol,Cyclohexane, Ethanol, Ethyl acetate, or water vapor.

18. The method of any of examples 16-17 wherein the sealing layercomprises a polymer material and a metal layer disposed between thepolymer layer and ambient and wherein the vapor generating material hasa pressure vs. temperature relationship that modulates the thermalconductance of the vapor in the gap between the enclosure plates.

19. The method of any of examples 16-18 and further comprising formingmultiple sets of such plates and bringing two sets together to create anenclosure with a chamber, and further comprising placing a device withinthe chamber such that is thermally insulated from ambient.

20. The method of any of examples 16-19 wherein including the vaporgenerating material in the gap comprises including a capsule of vaporgenerating material in the gap and breaking open the capsule followingdepositing the conformal sealing layer. Although a few embodiments havebeen described in detail above, other modifications are possible. Forexample, the process flows may not require the particular order shown,or sequential order, to achieve desirable results. Other steps may beprovided, or steps may be eliminated, from the described flows, andother components may be added to, or removed from, the describedsystems. Other embodiments may be within the scope of the followingclaims.

The invention claimed is:
 1. A thermal insulation device comprising; afirst plate; a second plate formed to nest adjacent the first plate witha gap between the first and second plates; a porous material disposed inthe gap between the plates; a sealing layer disposed between the firstand second plates such that the porous material is sealed from ambientat a pressure less than ambient; and a vapor generating materialdisposed in the gap.
 2. The thermal insulation device of claim 1 whereinthe vapor generating material has a pressure vs. temperaturerelationship that modulates the thermal conductance of the vapor in thegap between the enclosure plates.
 3. The thermal insulation device ofclaim 1 wherein the vapor generating material comprises cyclohexane,1-Butanol, Cyclohexane, Ethanol, Ethyl acetate, or water vapor.
 4. Thethermal insulation device of claim 1 wherein the first and second platescomprise a substantially planar portion and sides, wherein the sealinglayer is disposed between sides of the first, and second plates.
 5. Thethermal insulation device of claim 4 wherein the sealing layer comprisesa polymer material and a metal layer disposed between the polymer layerand ambient.
 6. The thermal insulation device of claim 5 and furthercomprising an additional layer of polymer and metal.
 7. The thermalinsulation device of claim 1 wherein the porous material is anano-porous material comprising an open cell material.
 8. The thermalinsulation device of claim 1 wherein the porous material is anano-porous material comprising a low-density mixture of fumed silica,fiberglass, and silicon carbide.
 9. The thermal insulation device ofclaim 1 wherein the porous material comprises a getter material.
 10. Athermal insulation enclosure comprising; a first plate; a second plateformed to nest adjacent the first plate with a gap between the first andsecond plates; a porous material disposed between the plates; a vaporgenerating material disposed in the gap; a sealing layer disposedbetween the first and second plates such that the porous material issealed from ambient at a pressure less than ambient; a duplicate set offirst and second plates having a porous material, vapor generatingaterial and sealing layer formed to mate with the first and secondplates to form a chamber; and a device disposed within the chamber thatis thermally insulated from ambient by the enclosure.
 11. The thermalinsulation enclosure of claim 10 wherein the first and second plates andduplicate first and second plates comprise a substantially planarportion and sides, wherein the sealing layer is disposed between sidesof the first and second plates, wherein the sides of each set of platesalign to form the chamber, and wherein the vapor generating material hasa pressure vs. temperature relationship that modulates the thermalconductance of the vapor in the gap between the enclosure plates. 12.The thermal insulation enclosure of claim 10 wherein the vaporgenerating material comprises cyclohexane, 1-Butanol, Cyclohexane,Ethanol, Ethyl acetate, or water vapor.
 13. The thermal insulationenclosure of claim 10 wherein the device within the enclosure comprisesa fuel cell based power generator and wherein the enclosure includes apath from ambient to allow ambient oxygen to reach the device.
 14. Thethermal insulation device of claim 10 wherein the sealing layercomprises a polymer material and a metal layer disposed between thepolymer layer and ambient.
 15. The thermal insulation device of claim 10wherein the porous material comprises an open cell material including alow-density mixture of fumed silica, fiberglass, and silicon carbide.